A resonant linear motor essentially comprises a linear motor, for example, a linear motor coupled to a resonant mechanism, which may comprise a spring, or to a to any load that produces a spring effect, to generate a resonant movement between the linear motor and the load. The applications of this type of linear motor may include driving fluid pumps in general, which can actuate variable loads.
Typical examples of this type of construction are linear motors employed on linear compressors that are often applied to cooling systems, due to its efficiency in terms of economy of electric energy. A linear compressor 100 employed on a cooling system is, as shown in FIG. 1, usually mounted inside a housing (not shown), the gas contained in this housing being under low pressure and being aspirated and compressed by the linear compressor for release in a high-pressure environment 7.
The gas compression mechanism takes place by axial movement of the piston 1 inside a cylinder 2 having a head 3; suction 3a and discharge 3b valves being positioned at the head 3, these valves regulating the entry and exit of gas in and out of the cylinder 2. The piston 1 is driven by a linear motor 10, which is formed by a stator 411 having a coil 11 and a support 4. The stator 411, in turn, actuates a magnet 5 that impels the actuator, in this case the piston 1, the actuator being associated to a helical-type spring 8, forming the resonant assembly of the linear compressor 100.
The resonant assembly, driven by the linear motor 10 that has the function of producing a linear reciprocating movement, causing the movement of the piston 1 inside the cylinder 2 to exert a compression action compressing the gas admitted by the suction valve 3a, to the extent where the latter can be discharged to the high-pressure side through the discharge valve 3b into the tubing 7.
The amplitude of the operation of the linear compressor 100 is regulated with the balance of the power generated by the linear motor 10 and the power consumed by the mechanism in compressing the gas plus other losses.
Another characteristic of the linear mechanism is the possibility of varying its pumping capacity, reducing the power of the electric motor, the operation amplitude in turn reducing the pumping capacity.
A parameter to be varied for controlling the amplitude of the linear compressor may be the feed voltage of the electric motor. From the feed voltage of the electric motor until the desired amplitude is achieved there are various coupled impedances, such as electric resistance of the electric motor, the inductance of the electric motor, capacitance if a capacitor is used, the counter-electromotive force, the impedances of the resonant system (mass/spring) and the compression work with its inherent losses. The impedance of this system depends upon the actuation frequency of the system, that is to say, the frequency of the voltage applied to the electric motor. At a certain frequency the output of this system is optimum, and this occurs when the mechanical system enters into resonance; at this frequency the output of the linear compressor is maximum.
“Gas Spring” Effect
The resonance frequency of the mechanism is not perfectly fixed. When compressed, the gas has a mechanical effect similar to the one of a spring (also known as “gas spring”), this “gas spring” is affected mainly by two factors: the distance between the piston and the valve plate and the pressures that the linear compressor operates.
The distance between the piston and the plate is altered when the piston stroke is reduced, generating an increase of the “gas spring” and in the resonance of the mechanism (this effect is the most relevant to the operation stability of the mechanism). In a cooling system, these two factors alter substantially, the pressures varying from the instant when the system is turned on until it reaches the operation condition, the operation condition is affected by the room temperature and the internal temperature of the cooler, the piston/plate distance is altered when the system needs more or less energy for its operation. In this way, the resonance frequency of the mechanical system varies due to various factors.